The limited epidemiological data available on borderline personality disorder suggest that the prevalence of the disorder is between .2 and 1.8 percent in the general community, 15 percent among psychiatric inpatients, and 50 percent among psychiatric inpatients with a diagnosis of personality disorder. No data on the incidence--the rate of new cases--of the disorder have been reported, and inferences about incidence based on prevalence rates are complicated by differences in the formal designation of personality disorders before and after DSM-III was issued. Current findings suggest that about 76 percent of borderline patients are female. Epidemiological study of borderline personality disorder has been hindered by the lack of a brief semistructured interview that can be used with large population samples and that does not require substantial clinical expertise. The authors discuss alternative research methods, including use of lay interviewers, recoding of existing data, telephone interviews, and self-report inventories.